Pro-Russian separatists listen to instructions near a machine gun during their service in eastern Ukrainian city of Donetsk on August 8, 2014.
A Ukrainian military spokesman on Sunday dismissed a call for a ceasefire by a separatist leader, saying this could only take place once rebels had shown "white flags" and surrendered.
Meanwhile the separatists stepped back from their earlier talk of a possible ceasefire and said the Ukrainian army had first to end military action.
Government forces on Sunday tightened the circle around the rebels' main redoubt, the big industrial city of Donetsk. Residents there reported heavy shelling from early in the morning.
A senior leader of the self-proclaimed Donetsk People's Republic had said on Saturday that the rebels were ready for a truce with government forces to allow humanitarian aid to be brought in.
Replying to a journalist's question on Sunday, military spokesman Andriy Lysenko said: "If there is this initiative, it should be carried out by practical means and not by words - by raising white flags and by putting down guns."
"We have not seen these practical steps yet," he said.
In a statement released later the rebels said they remained ready for a temporary truce to head off "a humanitarian catastrophe".
But they added defiantly: "As long as the Ukrainian army is continuing military action there can be no ceasefire."
© Thomson Reuters 2014